1. Field of the Invention
This invention relates to a numerical control method and, more particularly, to a numerical control method for application to a numerical control device for reading the numerical control data in a succeeding block during the execution of numerical control as defined by the current block.
2. Description of the Prior Art
The increased use of numerically controlled machine tools and the improvements which have been made in methods of creating numerical control (hereinafter "NC") data now make it possible to produce more sophisticated shapes by numerically controlled machining. Numerically controlled machine tools rely upon interpolation systems of two kinds. One is a linear interpolation system for interpolation along two or three axes simultaneously, and the other is a circular interpolation system for interpolation in XY, YZ and ZX planes, or in planes parallel thereto. In the machining a workpiece into a complex shape such as one having a sculptured (i.e., three-dimensional) surface or profile, the workpiece must be cut continuously by combining the linear and circular interpolation systems. It is also necessary to create the NC data required for such cutting. To this end, curves are approximated by a multiplicity of minute straight line segments or arcs, and NC data is created having one block, including a positional command, for each straight line segment. When a curve is approximated in this manner, the amount of movement specified by one block is extremely small, and may be on the order of only several microns in some cases. It follows then that very little time is required for machining or movement based upon one block of command data. This is particularly so when machining a workpiece made of a soft material such as wood, since cutting speed can be raised significantly.
When a very short time interval is required for movement based upon one block of data, a so-called prereading technique may be employed for reading data in such fashion so as to enhance machining efficiency. Before describing this technique, a conventional method that does not employ prereading is discussed.
In the conventional method, NC data is read in block-by-block, and each time machining or movement based on one block is completed, data from the next block is read in. The data read in followed by a format check, decoding, calculation of the specified amount of movement (namely an incremental value), and by other preprocessing, after which machining or movement is controlled based upon said next block. With this method, however, processing executed by the NC device does not keep up with the action of the machine tool because of such factors as preprocessing time and the response of the motor for driving the movable element of the machine tool, such as the table. The attendant disadvantages are reduced machining efficiency and a failure to attain a highly precise machined surface.
The abovementioned prereading technique is employed in an effort to solve these problems. With this technique, the next block of NC data is read in while numerically controlled machining based on the current block is being executed. This is known as "prereading" data. Thus, while numerically controlled machining based on the current or first block is in progress, preprocessing based on the next or second block may be performed in advance. Then, simultaneous with the completion of machining specified by the first block, there is a transition to machining based on the preprocessed data of the second block. Movement based on the second block may therefore be executed immediately without waiting for the completion of preprocessing following movement based on the first block. The result is a more efficient machining operation. Moreover, since control of movement specified by the second block can take place immediately upon completion of the preceding pulse distribution (interpolation) operation, curves, such as rounded corners, can be formed without "jerky" motion of the machine tool. Accordingly, the data prereading method affords the advantages of higher machining efficiency and greater machining precision.
Still, when approximating a curve by a multiplicity of lines or line segments wherein the time for movement specified per block is short, there are often cases where the preprocessing associated with a succeeding block is unfinished at the end of movement specified by the preceding block, even when the data is preread. This causes intermittent movement of the object being controlled, such as a cutting tool, resulting in diminished machining precision, damage to the machine tool, and prolonged machining time.